We have used the Hubble Space Telescope to directly measure the angular size of the large Kuiper belt object 2003 UB313. By carefully calibrating the point spread function of a nearby field star, we measure the size of 2003 UB313 to be 34.3$\pm$1.4 milliarcseconds, corresponding to a diameter of 2400$\pm$100 km or a size $\sim5$% larger than Pluto. The V band geometric albedo of 2003 UB313 is $86\pm7$%. The extremely high albedo is consistent with the frosty methane spectrum, the lack of red coloring, and the lack of observed photometric variation on the surface of 2003 UB313. Methane photolysis should quickly darken the surface of 2003 UB313, but continuous evaporation and redeposition of surface ices appears capable of maintaining the extreme alebdo of this body.

"After having some business dealings with men, I am occasionally chagrined, and feel as if I had done some wrong, and it is hard to forget the ugly circumstance. I see that such intercourse long continued would make one thoroughly prosaic, hard, and coarse. But the longest intercourse with Nature, though in her rudest moods, does not thus harden and make coarse. A hard, sensible man whom we liken to a rock is indeed much harder than a rock. From hard, coarse, insensible men with whom I have no sympathy, I go to commune with the rocks, whose hearts are comparatively soft."

"After having some business dealings with men, I am occasionally chagrined, and feel as if I had done some wrong, and it is hard to forget the ugly circumstance. I see that such intercourse long continued would make one thoroughly prosaic, hard, and coarse. But the longest intercourse with Nature, though in her rudest moods, does not thus harden and make coarse. A hard, sensible man whom we liken to a rock is indeed much harder than a rock. From hard, coarse, insensible men with whom I have no sympathy, I go to commune with the rocks, whose hearts are comparatively soft."

Yet again it seems to be only the term "planet" which attracts this debate. Consider neutron stars. They are not even as big as many asteroids or Kuiper belt objects, much less Pluto, yet no astronomer seems to be suggesting that their diminutive size means we should stop calling them "neutron stars" and dub them (say) "neutron objects" or "neutron dwarfs" instead.

======Stephen

Stars have a clear definition, in that they are capable or were capable of sustained nuclear fusion. So there are clear criteria here. The problem is that, despite what Britt thinks, the term planet is still largely pre-scientific, without clear criteria behind it.

QUOTE (nprev @ Apr 12 2006, 03:41 AM)

The IAU & everyone else has to face up to one fundamental fact: the objects in the Solar System exist along a continuum of sizes, from individual hydrogen atoms to Jupiter. Defining what is and is not a planet will always be a purely arbitrary convention by any objective standard, except for the apparent distinction that a planet has to independently orbit the Sun. Maybe it's time to throw out the concept entirely...?

Well, short of that heresy, maybe we just need to distinguish between "major" and "minor" planets. If that definition were adopted, I'd say that Mercury becomes the standard minimum body, and we have eight major planets. (Let's face it: it's embarrassing that Pluto is only half the size of the Moon!)

"Don't care what the latest rage is...Gimme the System of the sages...Living in the Middle Ages...It's good enough for me!

Yeehaw!

It may be rather hard to find, I haven't see in reprinted in ages, but look for Lester Del Rey's "The Sky is Falling", which I think was originally published in John. W. Campbell's "Unknown Worlds", the hard-fantasy companion to Astounding Science Fiction, sadly killed by WW-II paper shortages. There was a soft cover digest-mag sized reprint around 1960 in the US.

One classic scene has the resurrected engineer hero looking at a piece of the crystal sky-sphere that had fallen, with a glowing dot of star inside the crystal.

"Don't care what the latest rage is...Gimme the System of the sages...Living in the Middle Ages...It's good enough for me!

Yeehaw!

It may be rather hard to find, I haven't see in reprinted in ages, but look for Lester Del Rey's "The Sky is Falling", which I think was originally published in John. W. Campbell's "Unknown Worlds", the hard-fantasy companion to Astounding Science Fiction, sadly killed by WW-II paper shortages. There was a soft cover digest-mag sized reprint around 1960 in the US.

One classic scene has the resurrected engineer hero looking at a piece of the crystal sky-sphere that had fallen, with a glowing dot of star inside the crystal.

Actually, it doesn't work right now. The trouble is that some of those super-Mercurian iceballs may be in the Oort Cloud, and if so we'll NEVER know whether they exist there or not -- they're too far away for any conceivable type of direct observation. (The remarkable orbit of Sedna, which is pretty big itself, is suggestive.) Alan Stern suggested long ago that there's a real chance of some icy planetesimals out there bigger than Earth (indeed, we can't quite rule them out in the Kuiper Belt at this point).

Alan HAS suggested an alternative "scientific" definition for the minimum size of a planet -- namely, objects big enough to round themselves gravitationally. The trouble is that

(1) This definition, too, has seriously fuzzy edges -- consider Iapetus' distinct but not overwhelming departure from the spherical, and the fact that Proteus looks like a marshamallow. The last straw may have been the discovery of the remarkable rapid spin rate and resulting high elongation of 2003 EL61, which approaches Pluto's diameter on its long axis -- but is only half that on its short axis. Unlike the upper-size scientific definition of a planet -- an object too small to ignite deuterium fusion in its interior, as brown dwarfs do -- the "roundness" definition seems to me just too seriously shaky.

(2) If we do accept it, then there's also a veritable swarm of new objects which will have to be called planets, including at least four asteroids and a very big collection of KBOs. This may be acceptable to scientists, but the public will raise hell.

Combine these two factors, and I think that in the end we have to accept that the definition of "planet" will be unavoidably highly arbitrary. (2003 EL61 produces enough problems by itself; its long diameter is about 2000 km, which would fit my own proposal for defining a "planet" -- but its short axes don't.)

Alan HAS suggested an alternative "scientific" definition for the minimum size of a planet -- namely, objects big enough to round themselves gravitationally. The trouble is that

(1) This definition, too, has seriously fuzzy edges -- consider Iapetus' distinct but not overwhelming departure from the spherical, and the fact that Proteus looks like a marshamallow. The last straw may have been the discovery of the remarkable rapid spin rate and resulting high elongation of 2003 EL61, which approaches Pluto's diameter on its long axis -- but is only half that on its short axis. Unlike the upper-size scientific definition of a planet -- an object too small to ignite deuterium fusion in its interior, as brown dwarfs do -- the "roundness" definition seems to me just too seriously shaky.

But if you turn the definition on its head, and say that a minor planet is something that is small enough that it can stably retain a non-round shape, then you almost have a workable boundary, one that for "major" planets would exclude objects smaller than Iapetus or 2003 EL61 (and possibly even larger objects). And instead of adding in "a swarm" of small objects to the "major" planet category, the only marginal additions (at this date) would be Pluto, 2003 UB313, and possibly 2003 FY9. It would also place the boundary just around the 2000 km diameter measurement that has been proposed as well.

Of course, we might settle all this, and then when New Horizons arrives at Pluto, discover that it's actually shaped like a top hat with the crown pointing toward us.

I came up with a unique definition according to mass awhile back. Basically, I set the lower size limit for a major planet at the lowest possible diameter for a body with the density of water to have a mass at 0.001 Earth mass. Incredibly, the result came out to be a diameter of 2250 km! In this scheme, the only known objects in the Kuiper Belt that would qualify as major planets would be 2003 UB313 and Pluto. 2005 FY9 and 2003 EL61 are out! Later!

But if you turn the definition on its head, and say that a minor planet is something that is small enough that it can stably retain a non-round shape, then you almost have a workable boundary...

No, you've still got a seriously fuzzy one, given the question of what constitutes "round". (Iapetus? Proteus?) Ultimately, you HAVE to set some kind of arbitrary figures for the different dimensions of such worlds. (2003 EL61 presents a problem already, since it's almost as wide as Pluto on its long axis but only half as wide on its short axes. I suppose you'll have to make my "2000 km diameter" definition an object's minimum diameter along any axis to call it a planet.)

As if this issue hadn't already been faced on Earth with respect to what is a Mountain/Hill/Knoll or what is a River/Stream/Creek or what is a City/Town/Village.

In fact, the usages are quite arbitrary on Earth, depending upon geography, comparison class, and the particular language. There are mountains in Maryland that no one in western Colorado would even be able to perceive. I laughed when a New Yorker speaking Italian used "paese" to refer to Brooklyn. The natives of North America thought that the Allegheny/Ohio was a single river that was joined by the Monongahela where we now have Pittsburgh. The French agreed with them, while the English gave the Allegheny and the Monongahela equal status as sub-entities that joined to make the Ohio. (There, identity as well as category becomes an issue.)

The real heart of the "planet" problem is that scientists who often find the need for precise, hierarchical, mutually-exclusive categories come from the same species that found the "river" issue ultimately unimportant, or highly contextual and which yearns to do the same with the planet issue.

It's pretty useful to distinguish between viruses and bacteria, but I can't figure a reason why it is so important to distinguish between the category Pluto belongs to and the category Triton belongs to. Suppose we found such a body in another solar system that spent several orbits looping around a gas giant but would regularly be ejected into solo orbits until being periodically recaptured. Would we really want to say that Zelph is a planet and will remain so until Apr 22, 2078, when it will cease to be a planet for a span lasting until May 3, 2198?

Suppose an object were adjudicated to be 0.9999 the mass/diameter of an arbitrary cutoff until a new measurement added the needed quantity (to 1.0001) to cross the magic threshold. Would we want to say that it is a planet, defying our previous belief? The motivation for such a category eludes me.

Categories are a form of "data reduction"... a swirl of complex properties can be communicated with one handy label. They make sense when the world is chopped up that way. We've learned that nothing along the line of "planet" is so handy as we thought it was before (choose your date: 1801, 1978, 2003, 2005). What would it take to make a learned scientist realize that "planet" isn't a category like "bacterium" (scientifically useful), but is more like "river" (folksy and contextual)? A committee of longbeards is the last group who deserves to get their hands on this.

A committee of longbeards is the last group who deserves to get their hands on this.

::applause::

I vote that we use time-of-discovery as the cutoff, and anything discovered after 1950 is something other than a planet. Nine is hard enough on grade school kids, a few of whom think science should be something other than a bunch of mythological name tags to hang on orbiting objects.

While I am at it, I want one time zone PERIOD: UTC; and we all adjust our schedules accordingly. I reset my alarm three times trying to figure out when Venus Express was being placed into orbit, and still overslept

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